In polishing employing a polishing composition and a polishing pad, demands for a polished surface with high smoothness free from defects is increasingly high year by year. Along with such a tendency, the particle size of abrasive grains contained in a polishing composition becomes smaller. Further, in general, the particle size of abrasive grains is distributed, and if coarse particles which are extremely large relative to the aimed particle size are contained, such coarse particles will lead to surface defects such as scratches, and accordingly they should be removed.
Such coarse particles dispersed in a liquid medium are usually removed by a filter. In order to capture the coarse particles efficiently, it is desired to use a filter having a smaller aperture. However, the smaller the aperture of a filter, the more clogging is likely to occur. If clogging occurs, the efficiency in removal of the coarse particles tends to be deteriorated and finally, the filter will no longer be used. Accordingly, a filtration method with which clogging is less likely to occur, or a method of easily eliminating the clogging if occurred, has been desired.
From such a viewpoint, various filtration methods have been studied. Patent Document 1 discloses a filter for filtration of a fluid with an increased efficiency, by directly applying ultrasonic vibration to suppress an increase in the viscosity of the fluid, or by vibration, and a filter to which back pressure cleaning is easily conducted, if the filter is clogged, by directly applying vibration to the filter thereby to decrease the viscosity of a fluid with which the filter is clogged, or thereby to break the fluid by vibration.
Patent Document 2 discloses a liquid filtration method, which comprises subjecting a liquid to filtration while continuously or intermittently applying ultrasonic waves.
Patent Document 3 discloses a filtration apparatus comprising a filter such as a hollow fiber membrane, a nonwoven fabric, an open cell sponge, fibers or particles contained in a housing, wherein ultrasonic vibrators are respectively provided to two facing edge faces of the housing, at such positions that the phases of ultrasonic waves generated by the vibrators will not coincide with each other.
The above documents disclose to utilize ultrasonic waves for the purpose of improving the filter property, but failed to provide a method effective for a step of removing coarse particles (several hundred to several thousand nm) in polishing composition particles of from several ten to several hundred nm.